Do not reuse zmq context if we want to support fork()

ipykernel/kernelapp.py

@@ -229,7 +229,7 @@ def init_connection_file(self):
     def init_sockets(self):
         # Create a context, a session, and the kernel sockets.
         self.log.info("Starting the kernel at pid: %i", os.getpid())
-        context = zmq.Context.instance()
+        context = zmq.Context()
         # Uncomment this to try closing the context.
         # atexit.register(context.term)